With the development of micro-electromechanical technology, the digital photographing becomes more popular. The digital photographing can be applicable to record many matters, e.g. scenery and cultural activities, or to the secure monitoring.
Generally, the auto focusing (AF) is used the most in the digital photographing, because users would pay attention rather to desired matters, e.g. the composition of photo, than to the adjustment of focus. The AF has the contrast-detection AF and the phase-detection AF. The contrast-detection AF provides high focusing accuracy but has slow focusing speed. The phase-detection AF provides high focusing speed but has low focusing accuracy.
Even though the AF has many advantages, the manual focusing is more suitable than the AF in some photographic environments. For the manual focusing, a sharp frame that is watched by human eyes is a fact to determine whether a captured image is in focus or not, thereby leading to do the digital photographing inconveniently.